I reloaded an old file because I don't think there's another way. But technically it's easily possible? I did the cargo mission for Keirys and also went to Katari's planet and from there I just needed to get some other stuff so I went back and forth before I could do the pirates. There are a lot more planets available from the start tbh. Sithyrya only opens up after doing the pirates.
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Oh, I didn't realize it was possible to unlock the planets in the wrong order. Though, when I started playing, most of the planets weren't implemented at all, so it's possible I just unconsciously thought there was some order to them when there isn't. The walkthrough at least suggests there's an intended order.
Though, Urukai (at least currently) has the most difficult missions, so that kinda makes it feel like it's intended as a late game planet. Then again, Harmonia is definitely the easiest and it was implemented pretty late.
Yeaaaaah, well, I did have to restart a portion but I just made it to Harmonia, so all's well that ends well :') If anyone sees this and sits in the same boat as me: do the missions in order and don't go to random places lol
I haven't found a better way than training with Bradley yet, so can't confirm there's anything else before that!
Might also depend how you're planning to play. I went with lustful sub, which makes the game objectively more difficult, but does also mean you can skip 90% of combats if you don't care about getting experience.
When I started, I didn't initially realize combats are supposed to give experience since lust submit -- which is all I was doing -- doesn't give any, so all my experience was from missions until I was forced to do actual combat. It's entirely possible lack of experience was most of the reason I couldn't do much combat-wise. (It also meant I kept savescumming the pirate base to 100% avoid combat because I was unaware that you don't get sent back to the start if you win the combat.)